Reshape JSON declaratively — predictable, not a language. `mapping` mirrors the output shape; its leaf strings are dot-paths into `data` (a.b.0.c; negative index ok; 'contacts.*.email' projects over an array; '=text' escapes a literal). Objects/arrays nest; number/boolean/null are literals. Missing paths → null. Field rename/flatten/projection to fit a payload to a target schema. Distinct from /v1/jq (declarative vs query language).